Your Tang looks terribly skinny. I know you've only had him for a few days, what are you trying to feed him?
I use the algae sheets with garlic, the green sheets. They also come in red sheets, my tangs do not like the red sheets, I've also heard of Tangs ONLY eating the red sheets as opposed to the green sheets.
So if you're not having any luck getting him to eat one color, possibly try the other.
Tangs are grazers, in the wild they eat all day long, although the majority of what they're picking at on the rocks has very little nutritional value to them, but that's their habit, you need to try to get him to eat some algae, that's going to be his best chance for survival.